The present application relates to NMR spectroscopy and imaging and, more particularly, to novel apparatus for converting single-species nuclear magnetic resonance (NMR) spectrometry/imaging apparatus into multi-species spectrometry apparatus.
The potential for use of NMR spectroscopy for in vivo medical diagnosis of human patients is well known. At present, the vast majority of NMR spectroscopic imagers operate with a single nuclear species, e.g. hydrogen, or .sup.1 H, nuclei. In a typical high-amplitude static magnetic field system, such as described and claimed in co-pending U.S. patent application Ser. No. 743,125, filed June 10, 1985, assigned to the assignee of the present application and incorporated herein in its entirety by reference, a super-conducting magnet with a static main magnetic field of about 1.5 Tesla (T) is utilized. As is well known, the resonance frequency F of the nuclear species (e.g. .sup.1 H) to be investigated is given by the Larmor formula: F=.gamma.B.sub.O, where B.sub.O is the magnitude of the main static magnetic field (in Tesla) and .gamma. is the gyromagnetic ratio for the particular nucleus species. For a main static field B.sub.O of 1.5 T and .gamma..sub.1H of about 42.58 MHz/T, the resonance frequency F is about 63.87 MHz. It is advantageous to acquire NMR spectroscopic image information by irradiating the sample, to be investigated, with an excitation signal having a pair of signal portions in phase-quadrature with one another, and to provide a received response signal, from the sample, separated into a pair of phase-quadrature components. It is therefore highly desirable to utilize Zero-Intermediate-Frequency (ZIF) techniques to generate the in-phase (I) signal and the quadrature-phase (Q) signal in both the transmitting and receiving portions of the NMR spectrometer. The use of such ZIF techniques requires that at least one 90.degree. phase-shift-network (PSN) be utilized for quadrature-modulation in the excitation transmitter and that another 90.degree. PSN be used in quadrature-detection apparatus in the response signal receiver. It is not prohibitively difficult (and is, in fact, rather easy) to design a 90.degree. PSN to operate with a desired low magnitude of both phase and amplitude unbalance errors over a relatively narrow bandwidth centered about the single Larmor resonance frequency for a single-species spectroscopic imager. However, it is prohibitively difficult to provide the same extremely low phase and amplitude unbalance error magnitudes in a practical NMR spectrometer for imaging more than one of the nuclei species involved with living organisms. For example, in addition to .sup.1 H hydrogen nuclei, it may be desirable to compare spectra from .sup.13 C carbon (having a gyromagnetic ratio .gamma. of about 10.71 MHz/T, and a resonance frequency F of about 16.065 MHz. in the aforementioned 1.5 T system) and/or .sup.31 P phosphorous (having a gyromagnetic ratio .gamma. of about 17.23 MHz./T, and a resonance frequency F of about 25.845 MHz. in the same 1.5 T system). Investigations requiring images of the respective abundance of these three nucleus species will thus require phase-shift networks operating over a two octave bandwidth (i.e. a 4:1 frequency span) with the required low magnitude of phase and amplitude unbalance errors; such phase shift networks are presently impractical. It has been suggested to utilize a different set of phase-shift networks, each optimized for an assigned nuclear species resonance frequency, and select and switch the proper network into the excitation signal transmitter and into the response signal receiver, as the need arises; this solution has not only proven to be relatively costly, but also not particularly useful in the highly-automated spectroscopic imaging environment which should be provided in spectroscopy apparatus for use by life science professionals. Accordingly, apparatus for enabling a single-species NMR spectroscopic imager to operate at the resonance frequencies of a plurality of different nucleus species is highly desirable.